The paramount objective of any audio or video coding technique is the ability to reconstruct the signal such that a human observer detects only minor differences, and preferably none, with respect to the original (non-coded) signal. For example, a good audio compression technique may take a high-quality representation of the audio signal (like a 44.1 kHz PCM representation found on commercial audio compact disks or CDs) and convert it to a more compact representation that requires fewer bits than the input representation. From this compact representation, one can reconstruct an approximation to the original high-quality representation. The process of compacting or compressing the audio signal can involve "lossless" (also referred to as "noiseless") techniques that preserve all the information in the input signal and/or "lossy" (also referred to as "noisy") compression techniques that discard information, particularly less important information, in the input signal.
Current lossy compression techniques are typically based on a model of the signal source or a model of the receiver (which is generally the human ear or eye) [Jayant, '93]. A vocoder, which uses a model of the vocal tract to compress speech signals, is an example of a source model-based compression technique. A perceptual coder, which uses models of human psychoacoustic thresholds (for example, masking thresholds), is an example of a receiver-based model.
The prior art, which incorporates methods related to "psychoperceptual audio coding," or simply "perceptual audio coding," techniques, exploits the auditory masking properties of the coded signal to at least partially mask or conceal the coding noise generated by the compression process. Such coders generally divide the frequency spectrum of the signal into psychoacoustic "critical bands", and compute a masking threshold, for each critical band, based on the properties of the signal in the current band as well as adjacent bands (termed "spread of masking"). Based on the computed masking thresholds, these perceptual audio coding methods perform compression by re-quantizing the signal such that the resulting coding noise, in each band, is close to or below the computed masking threshold. The presently disclosed method can be described as a perceptual approach to audio coding, however, the disclosed method does not make use of psychoacoustic critical band techniques or masking threshold computations. Rather, the presently disclosed method advantageously incorporates concepts related to neurophysiological information capacity and makes use of techniques that quantify the compressed signal fidelity necessary to reproduce a neural representation in the brain that is similar to that produced by the original signal.